Recently, engines with high power and increased fuel efficiency have been configured in order to reduce CO2 emissions. Therefore, valve sheets for internal-combustion engines have been used in such a severe environment as a high temperature and a low lubrication, and various approaches have been made.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2006-299404 proposes a material which includes hard particles of 10 to 60% by weight in a matrix phase, wherein the matrix phase contains 0.3 to 1.5% of C and of one or two or more selected from Ni, Co, Mo, Cr, and V in a total amount of 1 to 20%; and the hard particles have a composition which includes one or two or more among an intermetallic compound containing Fe, Mo, and Si as main components, an intermetallic compound containing Co, Mo, and Si as main components, and an intermetallic compound containing Ni, Mo, and Si as main components, which includes 1 to 15% of Si and 20 to 60% of Mo, which includes 10 to 70% of one or two or more selected from Cr, Ni, Co, and Fe, and of which the remaining portions are Fe and incidental impurities; and have a Vickers' hardness of 500 HV 0.1 to 1200 HV 0.1: has a density is 6.7 g/cm3 or more: and has a radial crushing strength of 350 MPa or more.
In addition, JP-A No. 2004-307950 proposes an iron-based sintered alloy obtained by dispersing 3 to 20% by mass of hard particles relative to the total mass of the matrix in a matrix containing 3 to 12% of Ni, 3 to 12% of Mo, 0.1 to 3% of Nb, 0.5 to 5% of Cr, 0.6 to 4% of V, 0.5 to 2% of C, Fe, and incidental impurities.
In addition, in JP-A No. 2004-156101, it is proposed that hard particles include 20 to 70% by weight of Mo, 0.2 to 3% by weight of C, 1 to 15% by weight of Mn, and Fe, incidental impurities and Co as the remaining portion; and that the sintered alloy has overall components including 4 to 35% by mass of Mo, 0.2 to 3% by mass of C, 0.5 to 8% by mass of Mn, 3 to 40% by mass of Co, and incidental impurities and Fe as the remaining portion; where the base component includes 0.2 to 5% of C, 0.1 to 10% of Mn, and incidental impurities and Fe as the remaining portion, and the hard particle component includes 20 to 70% of Mo, 0.2 to 3% of C, 1 to 20% of Mn, and incidental impurities and Co as the remaining portion; and the hard particles are dispersed in the base in an area ratio of 10 to 60%.
In addition to the aforementioned patent documents, there have been many disclosures in the related technical field. However, any disclosure concerning characteristics other than chemical components regarding a powder for forming a valve sheet is not found. The inventors have been confronted with an incompatible problem that the powder needs to be softened so as to improve the compactibility of a powder for the sintered valve sheet made of an iron-based alloy and the powder also needs to be hardened so as to improve the abrasion resistance. The reasons are as follows.
First, in addition to high strength, the valve sheet is required to have good thermal conductivity so as not to store heat of the combustion in the engine in the valve sheet itself. Therefore, the sintering density needs to be high. In order to increase the sintering density, the density of the compressed powder before the sintering needs to be high. In order to increase the density of the compressed powder before the sintering, the compactibility at the time of the compression molding needs to be good. In order to increase the compactibility, the hardness of the powder needs to be decreased.
However, if the hardness of the powder is decreased, the hardness of the valve sheet that is the final product after the sintering is decreased, so that the abrasion resistance deteriorates. In addition, for makers of sintered parts of the valve sheets, it is feared that when a carbide having different deformability from the metal is precipitated so as to increase the abrasion resistance, the counterpart may be abraded.